The present invention relates to electrical machines, and more particularly to brushless so-called xe2x80x9cdiscoidxe2x80x9d machines comprising a rotor and a stator placed facing each other on the axis of rotation of the rotor.
The invention relates to a synchronous machine comprising at least two rotors placed on either side of a stator. The use of two rotors makes it possible to double the torque for substantially the same quantity of iron and copper in the stator, thus making it possible to improve efficiency quite considerably. The rotors comprise permanent magnets disposed between pole pieces suitable for concentrating their magnetic flux.
U.S. Pat. No. 5,245,238 describes a known machine of that type, Its windings are distributed.
In the machine of the invention, the stator has a magnetic circuit with teeth, each serving as a core for a coil. This makes the stator easier to manufacture and the overall size of the machine can be reduced because there is no winding overhang of the kind to be found in conventional machines having distributed windings.
In a preferred embodiment, at least two of said rotors are angularly offset relative to each other. Such an offset makes it possible to reduce or even eliminate ripple in the torque.
In particular, when the number m of phases is even, at least two of said rotors can be offset by an angle xcex1 close to xcfx80/S, where S=m.p gives the number of teeth of the stator, p being the number of pairs of poles. When m is odd, then at least two of said rotors can be offset by an angle xcex1 close to xcfx80/2S.
The number of stator teeth S can also satisfy one of the following relationships:
S=2m.n where p=m.nxc2x11, n being an arbitrary integer; or
S=m(2n+1), where 2p=m(2n+1)xc2x11, and n is an arbitrary integer.
In particular when S satisfies one of these two relationships, the two rotors need not be angularly offset.
In a particular embodiment, the stator does not have a magnetic yoke, thus making it possible further to reduce losses to a significant extent, particularly at high speeds. The absence of a stator yoke eliminates those losses which would have taken place in the stator due to the magnetic induction varying in time. The coils and the teeth of the stator can then be housed in a non-magnetic casing. The same applies to the magnets and the pole pieces of the rotors. The stator may have teeth presenting facing faces that are plane and parallel, at least over a major fraction of the length of each tooth.
The stator teeth may be supported by two non-magnetic pieces connecting them to the casing.
Each rotor pole piece may have a rounded surface facing the stator, in particular a surface that is generally convex towards the stator, so as to reduce harmonics in the electromotive force.
In a particular embodiment, the magnetic circuit of the stator is made from a substance based on a powdered magnetic material. The pole pieces of the rotors may also be made from a substance of the same type, where appropriate, thereby facilitating the provision of rounded surfaces directed towards the stator.
The invention applies equally well to providing a motor or a generator.
The invention also provides the use of a machine as defined above as a motor or as a generator and rotating at a speed in excess of 4000 revolutions per minute (rpm), for example.